For many years, the catalysts for ammonia oxidation have been formed of meshes or gauzes of platinum or its alloy with other precious metals.
Such catalysts have good activity and selectivity but suffer from the disadvantage that the catalyst is not only very expensive but also it exhibits an appreciable loss of platinum at the high temperatures of the oxidation reaction with consequent low catalyst life, which requires frequent replacement.
It is therefore desirable to provide a replacement of such precious metal catalyst.
It is well known that oxides of metals such as manganese, iron, nickel or, especially cobalt, often used in conjunction with one or more rare earth metal oxides, exhibit activity for ammonia oxidation.
CN-A-86/108 985 describes a catalyst composition of formula La1-XCeCoO3 (where x is a number from 0 to 1) having perovskite structure, endowed with good activity and selectivity when tested on small scale, which decreases when operating at temperatures (800°-1000° C.) normally employed for ammonia oxidation.
U.S. Pat. No. 4,963,521 describes an exhaust gas catalyst which, in one embodiment, is formed of honeycomb cordierite coated with a first layer of gamma alumina mixed with minor proportion of zirconia and ceria, and a second layer formed of cobalt oxide or platinum, rhodium and palladium.
No mention is made that the catalyst can be used for oxidizing ammonia.
U.S. Pat. No. 5,217,939 describes an ammonia oxidation catalyst obtained by coating a reticulated foamed ceramic or metal substrate with cobalt oxide or with a noble metal.
The coating is obtained by immersion of the foamed substrate in a solution of a carboxylate of cobalt, or of a noble metal, removing the substrate from the solution and calcinating at temperatures from 260° to 800° C. (at which the cobalt carboxylate is converted into oxide and the carboxylate of the noble metal is reduced to the metal).
The ceramic foams have a number of pores per linear inch of 10 to 100 (4-40/cm); 30 pores in the examples.
The conversion of ammonia to NO using the foam-Co oxide coated is 92-95%; using the foam coated with Pt 97-100%.
Further processing is necessary to convert nitric oxide (NO to NO2) and then to nitric acid.
More efficient and economical catalysts in the production of nitric acid are therefore needed.
More efficient catalysts producing relatively high yields of NO2 are described in U.S. Pat. No. 5,690,900. The catalysts are formed of porous ceramic (200-600 cells per square inch i.e. 14-24 cells per linear inch=5-9 cells/cm) coated with at least three layers: the first is formed of alumina with minor proportion of ceria and zirconia, the second of oxides of cobalt, zirconium and cerium, the third of platinum metal.
The layers are obtained by immersing the porous ceramic body in a suspension of alumina, zirconium oxide and cerium nitrate, removing the impregnated ceramic structure and calcinating at 600° to 1000° C.
The resulting surface area is 80-120 m2/g.
The thus coated ceramic is then immersed in a solution of cobalt acetate, cerium nitrate and zirconium acetate, removed from the solution, and calcinated at 600-1000° C.
The last layer is obtained by immersion in a solution of platinum oxalate, and calcination at 600-1000° C. of the structure removed from the solution of the previous treatment.
The ammonia conversion to NO/NO2 is 95-100% with NO/NO2 ratio of 75/25 to 60/40.
U.S. Pat. No. 4,820,678 describes honeycomb-like alloy tapes coated with a catalyst having the perovskite (ABO3) or spinel (A2BO4) structure, wherein A comprises cations of rare earth metal, B, in the perovskite structure, is selected from manganese, copper and nickel cations, in the spinel structure is selected from iron and nickel.
The catalyst is used for the purification of industrial waste gases, exhaust gases from automobiles, and purification of air.
The honeycomb tapes are obtained by perforating Fe—Cr—Al or Fe—Ni—Al alloy strips of about 0.05-0.12 mm thickness at distance of about 1.1-1.2 mm apart to form small holes of 0.4×0.4 mm.